Greenhouse heating

[petershaw]

I dont mean to be a pain, but with winter coming or already here it seems the thread has evolved into anything but.

Is there someway to move all of the non-heating and energy conservation comments to another thread?

Always looking for new ideas on heating and retaining heat even in my "mild" climate and solar panels.

Peter

[Martin Bohnet]

Actually, the topic started to fill in a function every living forum needs - something like a pub or bar talk area, with jumping topics and a sort of camaraderie  between the more frequent participants - i mean it was never really meant to be about greenhouses as it started out as a political comment on the mess Europe has found itself in by getting to dependent of the resources of an autocratic system.

I'd recommend to leave the pub as it is and start a new technical oriented topic about Greenhouses if you want that kind of discussion. - and do so in the off topic general plants and gardening, not in the really off topic off topic Area.

[David Pilling]

Peter - if you go to the top left, where your name appears when logged in, then you can visit 'ignore boards' and opt out of the far off topic stuff.

Really how to heat your greenhouse is an interesting subject, one that has been touched upon over the years in the PBS list and worthy of discussion.

[CG100]

Just by way of information..............

My collection was started again a couple of years ago, so is not huge - overspilling a 12 x 3 bench now, but I was determined not to heat the entire greenhouse, as mentioned in the thread referred to.

I have rehashed everything properly (last minute, only a week or two ahead of frosts).

The greenhouse long axis is north-south, so the great majority of light comes in via the south end and roof.
I have double bubble-wrapped the west- and north-facing walls from just below bench level to the eaves, and single bubble-wrapped the south end (my greenhouses sit on one course of concrete walling blocks, so are an extra 10 inches (?) taller). The east side of the bench is the path within the greenhouse.
At eave level, I have secured some 3 foot wide wire netting, as a "roof" over the bench, supported on some 8mm studding and small battens (lengths of broom handle).
I have stretched single lengths of stout wire at eaves level, againt the wall, and along the front of the wire netting.
A length of bubble-wrap hangs on the front wire, like a curtain.

When the forecast is for under 5C, I "draw the curtain", and lay a length or more of bubble-wrap on the wire netting. It isn't "air-tight", but everything overlaps so there are no huge leaks.

I bought a couple of plug-in energy "meters", and the 12 x 3 x 2.5 foot cloche used 4.5kWhr of electricity to maintain 6C difference to ambient over a period of 15 hours (5C in the cloche, average -1C outdoors (from local Met Office weather station data and probably a little cooler here, maybe an extra degree, as I am outside of town)). So, around 0.3kWhr per hour. Two consecutive nights were very similar, both in average temp. and in energy usage - I was very pleased, and amazed.

I could not find what I thought was near enough to an ideal (fan) heater, so made some. The cloche has 2, running at 250W, but switchable to 500W if the weather gets very cold. The fans run so long as the greenhouse temperature is below 6C, in other words, always circulating when the weather is cold. The fans run at ~20W.

[David Pilling]

Weather Underground is a useful source of more local weather data, here's a link to the weather station on the other side of the avenue I live on:

https://www.wunderground.com/dashboard/pws/ITHORNTO12

I know you're not interested in the weather here but it shows the neat display of where the weather stations are. Also it is not limited to the UK.


Bubble wrap seems popular in the UK for lining greenhouses in the Winter. The temperatures are probably around the sweet spot for it. Once in a while I'll get a very large piece for free around a parcel.


Typical UK price is £0.30 for a kilo-Watt hour of electricity, the determined reckon on £0.02 for a diesel fuelled heater off ebay.

(They probably using potato chip (French fries) oil, so all those carbs will keep you warm too).

[CG100]

Weather Underground is a useful source of more local weather data

I have not searched in detail, but the locations look to be the same as the Met' Office use. If you use the Met' Office website, you put in your postcode and a list of local weather stations appears on a drop-down menu  and you choose which one you want, which appears to be the same.

Bubble wrap seems popular in the UK for lining greenhouses in the Winter. The temperatures are probably around the sweet spot for it.

I am unsure how there can be a "sweet spot" for insulation of a greenhouse when bubble-wrap costs peanuts, assuming that you don't throw it away every year, or is free. I suspect that insulating the bench cost around £4-5, I have not worked things out in detail.
I only have to save something like 15kWhr and I am ahead. I am sure that there will be U values for bubble-wrap online somewhere, but life is far too short.
The heat lost from the insulated bench, into the bulk of the greenhouse - 8 x 12 - raised the greenhouse temperature by about a degree, assuming all that heat came from the insulated bench, ignoring any contribution from the concrete floor etc.

the determined reckon on £0.02 for a diesel fuelled heater off ebay.

Diesel, on full combustion, liberates around 44MJ/kg.
One kWhr is 3.6 MJ.

So 1kg diesel when burnt liberates a little over 12 kWhr of energy. Diesel is close to a density of 1, so, near enough, 1kg is 1 litre. A litre of disel is around £1.45, so IF all the energy could be used, heating using diesel would cost around 12p per kWhr - more expensive than domestic mains gas.
However, two problems - you need ventilation (an oxygen supply) for combustion, you do not for electric heating, and ventilation means lost heat, also the combustion products must be disposed of, which also means lost heat.

Modern household boilers burning gas, all singing, all dancing, can get around 90% efficiency. (Even old gas boilers achieve over 80% efficiency. Which is why the economics of changing an old for a new boiler, so long as the old one works, do not stack up. It will depend on fuel prices, but generally, the payback period for the new boiler is longer than the expected life of the new boiler.)

Diesel is pretty close, chemically/in terms of combustion, to central heating fuel. I have never heard anyone suggest that oil central heating was a fantastically cheap option in the UK.

[David Pilling]

On weather stations I beg to differ. The link I gave goes to a map that can be zoomed to street level and the weather station belongs to a neighbour I know, he bought it, he maintains it and it is nothing to do with the Met Office. I believe a fellow PBS person has their garden weather station in California on Weather Underground.

On diesel, nice analysis, the laws of Physics do not always apply to You Tube videos. There is tax on road fuel diesel and heating oil is less than half the price. Chip oil is involved. One source of irritation on the road are large lorries approximately emblazoned with "this vehicle is powered by waste vegetable oil".

In the latest video the proponent of diesel heaters puts lots of effort into extracting heat from the exhaust gasses. If anyone is interested this is the video:

https://www.youtube.com/watch?v=gNKTZosL6d8

665K views...

[gastil]

My weather station data gets shared to a data-distributor. I think it might be PWS or AWS. Then lots of systems subscribe to that data distributor. So my little amateur weather station does show up on public systems, not just weather underground. They have rating systems for data quality that look at how long the station has operated continuously, how consistent it is with nearby stations, and other stuff. But apparently their bots do not read my comment field where I state it is too close to trees and not at regulation height. Also my rain bucket sometimes jams or gets filled with bird poo. So when you get free data... caveat emptor. 

I also have a soil moisture (tension) and soil temperature meter about 5" deep in the sand plunge underneath the weather station. And I have a leaf wetness meter. I used to gaze at graphs showing the damped oscillation of soil temperature versus air. Not just damped, but phase delayed. 

The one thing my Rainwise station does not do well is summon rain when we need it. 

[CG100]

On weather stations I beg to differ. The link I gave goes to a map that can be zoomed to street level and the weather station belongs to a neighbour I know,

That maybe the case with you.
I could see nothing but the same points close to me as used by the Met' Office.

I am unsure if the Met' Office actually maintain very many weather stations themselves - two near here are airports, one of them military.

The diesel figure was never going to be correct at 2p/kWhr.

Presumably central heating oil has escaped the application of VAT that now applies to red diesel (presumably, red diesel no longer exists)? If so, central heating oil will probably now be the prefferred choice of van drivers who won't pay for fuel at "full" price?

I just buy vehicle fuel - I need it, so buy it, so have no idea what they are in the UK now, except that petrol contains several % ethanol.
I wonder where all the spent oil from commercial premises went before conversion to diesel substitute was an option?

As a point of interest, which surprised me when I did some digging after someone asked, to make vegetable oils compatible with diesel engines, it has to be partially hydrolised, which generates glycerol as a by-product. (All natural fats are tri-glycerides - three fatty acids attached to one glycerol molecule).

I have no idea what is done to vegetable oils to make it burn like Jet A1, quite possibly the same thing?

[David Pilling]

Red diesel is ordinary diesel with the addition of a dye, it is taxed at a lower rate in the UK and is legally only permitted to be used for limited purposes. It still exists in 2023 although where it is allowed to be used has just become more limited.

Gastil - another thread needed "making it rain" - my money is on bottle rockets fuelled with hairspray...

[CG100]

it is taxed at a lower rate in the UK and is legally only permitted to be used for limited purposes. It still exists in 2023 although where it is allowed to be used has just become more limited.

Another name was agricultural/tractor diesel, or gas oil (which is what it actually is), but it was also permitted for use in stationary and boat engines. It isn't actually dyed diesel, but it is dyed, and close enough for most, but not all, applications that it can be used instead of white (fully tax-paid) diesel.

I was "assured" that it would very shortly disappear something like 5 years ago, by someone who owned a canal barge, which travel something like 4-5 miles per gallon. He implied that it was an end to all red diesel.

Looking at current law, it seems that the legal users are very largely still legal, although boat engines are restricted to marine (which is presumably correct, as in salt water only), and commercial space heating isn't allowed either.
Around here, the canal marinas were THE source for hooky diesel for vans.

[Bern]

Always looking for new ideas on heating and retaining heat even in my "mild" climate and solar panels.

Really how to heat your greenhouse is an interesting subject, one that has been touched upon over the years in the PBS list and worthy of discussion.

I propose renaming this post as Greenhouse Discussions Only and moving it to General Plants and Gardening.  Is that OK with you Peter?  And David, if it is OK and worthwhile, can it be done without too much ado?  The Heating Your Greenhouse posts can then continue as is and ad hoc. 

[illahe]

Hi Peter, 

Have you looked into these phase change tiles? They seem to offer a higher btu benefit than passive solar capture like water barrels in in a much smaller footprint. You could line a greenhouse bench or insulate a sun gathering wall with them. I'm hoping to give them a try in my new high efficiency climate battery greenhouse design i'm working on. link here: phase change tiles

I have a few small kerosene heaters that can burn diesel and employ those if the temps look extreme enough. Considering a propane heater as well, but with the cost of propane it's going to be rough if we get a tough winter. Fortunately for the plants, this winter has been very mild so far with temps about 9 degrees above average for December. Unfortunately, it also means no snow in the mountains. 

Mark

[Robert_Parks]

Hi Peter,

Have you looked into these phase change tiles? They seem to offer a higher btu benefit than passive solar capture like water barrels in in a much smaller footprint. You could line a greenhouse bench or insulate a sun gathering wall with them. I'm hoping to give them a try in my new high efficiency climate battery greenhouse design i'm working on. link here: phase change tiles

Interesting! A quick browse shows relatively high temperature phase change, do they come in lower temperature ones for keeping a greenhouse over freezing? And what is the approximate price per 2'x2' panel?

[CG100]

I wish I knew where it is, but Kew did lots of measurements over several years across various sites and published figures to calculate how much heat any greenhouse, anywhere in the UK, would use if kept at temperature X. This was back in the 1960's? 1970's?

There are calculators online, but compared to the Kew one they must be pretty crude - the Kew one required figures for floor area, glass area, location (as in where in the UK, so allowing for local climate and day length), and minimum temperature required. It also allowed a figure for leaks. 
It was compiled before anyone seriously bothered with insulation but the number would still be close enough.